Mattress border



FIG. 1 is a front and side view of a mattress border, showing my new design;

FIG. 2 is a side view of the mattress border;

FIG. 3 is a fragmentary side view of a mattress border on an enlarged scale; and,

FIG. 4 is a front view of the mattress border.

The broken lines in the drawings are for illustrative purposes and form no part of the claimed design. 

CLAIM The ornamental design for a mattress border, as shown and described. 